Debt - Unsecured Term Loan Facilities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Total Principal	$ 1,542,839
Long-term Debt, Total	1,533,641
Unsecured Debt [Member]
Total Principal	0	$ 240,000
Unamortized debt issuance costs	0	(2,151)
Long-term Debt, Total	0	237,849
Unsecured Debt [Member] | 2023 Term Loan [Member]
Total Principal	0	40,000
Unsecured Debt [Member] | 2024 Term Loan Facilities[Member]
Total Principal	0	100,000
Unsecured Debt [Member] | 2026 Term Loan [Member]
Total Principal	$ 0	$ 100,000